Method for production of porous cross-linked polymer sheet

ABSTRACT

A method for dehydrating a porous cross-linked polymer conveniently to a low final water content is provided. A porous cross-linked polymer sheet is produced by a method which comprises causing a porous cross-linked polymer obtained by forming and polymerizing a water-in-oil type higher internal phase emulsion to be dehydrated by the use of non-woven fabric rolls furnished with an aspiration mechanism. Properly in this case, the porous cross-linked polymer is subjected to preliminary squeezing. In accordance with the present invention, a porous cross-linked polymer abounding in a water absorbing property to be dehydrated to a low final water content with a small number of rolls.

TECHNICAL FIELD

This invention relates to a method for producing a porous cross-linkedpolymer sheet of an extremely small water content by dehydrating with anon-woven fabric roll a porous cross-linked polymer of a prescribedthickness obtained from a water-in-oil type high internal phase emulsion(hereinafter occasionally referred to simply as “HIPE”).

BACKGROUND ART

As a technique for obtaining a porous substance formed of uniform opencells of a minute diameter, a method for producing a polymer bycross-link polymerizing an HIPE in the presence of a specific surfactanthas been known. It is generally held that the term “HIPE” used hereinrefers to an emulsion so formed that the disperse phase accounts for aratio exceeding 70 vol. % based on the total volume thereof [K. J.Lissant, Journal of Colloid and Interface Science, Vol. 22, 462 (1966)].U.S. Pat. No. 5,334,621, for example, discloses a method for producing aporous cross-linked polymer by using such an HIPE (hereinafter referredto simply as “HIPE method”).

This HIPE method produces a porous cross-linked polymer by preparing anHIPE containing (i) a polymerizing monomer mixture containing anoil-soluble vinyl monomer and a cross-linked monomer possessed of notless than two functional groups in the molecular unit thereof, (ii) awater phase accounting for 90 mass %, preferably 95 mass %, andparticularly preferably 97 mass % of the emulsion, (iii) a surfactantsuch as a sorbitan fatty acid ester and a glycerol mono-fatty acidester, and (v) a polymerization initiator and heating the HIPE till itpolymerizes and forms a cross-link. According to this HIPE method, aporous cross-linked polymer formed of open cells in a reticular patternis produced by virtue of reversed-phase emulsion polymerization. Theporous cross-linked polymer which is obtained by the HIPE method,therefore, is possessed of such characteristic properties as lowdensity, water absorbing property, water retaining property, heatinsulation and soundproofing property.

The porous cross-linked polymer which is produced by the HIPE methoddescribed above, however, is disposed during the process of productionto expose the formation of cells to the influence of the ratio of thewater phase, i.e. an internal phase to the oil phase, i.e. an externalphase, namely W/O, during the reversed-phase emulsion polymerization. Aneffort to obtain a porous cross-linked polymer having as large a voidvolume ratio as permissible inevitably results in increasing the waterphase side numeral of the ratio W/O. The porous cross-linked polymer ofthis quality enjoys a large demand. The reason for this large demand isthat when the porous cross-linked polymer has a large void volume ratio,it excels not only in absorbing property but also in heat insulatingproperty and sound insulating property and, therefore, finds utility invarious fields such as building materials, audio products, andhorticultural articles. U.S. Pat. No. 5,334,621 mentioned above, forexample, has a statement that the porous cross-linked polymer, whenproduced by the HIPE method, particularly preferably has a density of 97mass % (W/O=33/1). Even generally, the W/O is actually attained moreoften than not in the range of 30/1-100/1. Specifically, an attempt toproduce a porous cross-linked polymer by the HIPE method necessitates alarge volume of water for the formation of an HIPE. This fact impliesthat when the porous cross-linked polymer is produced by the HIPEmethod, the produced porous cross-linked polymer is fated to containwater and, therefore, is required to be dehydrated and dried.

The dehydration of the porous cross-linked polymer which has beenobtained by the polymerization of an HIPE is effected, as demonstratedin Example 2 cited in the official gazette of National Unexamined PatentPublication 2000-500,796, by nipping the polymer between paper towelsand slowly squeezing it till the aqueous phase is removed.

The official gazette of National Unexamined Patent PublicationHEI-11-503,177 discloses the dehydration which is effected bycompressing an HIPE foam to expel the residual water therefrom orsubjecting the foam and the water lodged therein together to atemperature in the approximate range of 60° C. to 220° C. or a microwavetreatment, to vacuum dehydration, or to the combination of compressionand thermal drying/microwave/vacuum dehydration. This compressivedehydration is accomplished by the compression which is produced with aseries of paired porous nip rolls provided with a vacuum unit adapted todecrease the amount of the residual water phase to about three times themass of the monomer which has been polymerized.

In the official gazette of WO86/06,766 is disclosed a liquid absorbingdevice which is used for removing a liquid from a given object fortreatment containing the liquid. Specifically, this device is formed bywrapping a felt of excellent initial liquid absorbing power around thesurface of a liquid absorbing roll and combining the liquid absorbingroll with an aspiration mechanism and is consequently enabled to utilizeeffectively the capillary effect originating in the numerous poresinherently possessed by a fibrous sheet, smooth the transfer of liquid,and exalt the liquid absorbing power and the property of retaining theliquid absorbing power.

The porous cross-linked polymer obtained by polymerizing an HIPE,however, has a high water content and is deficient in strength. When thefoam and the porous cross-linked polymer obtained by polymerizing anHIPE which are disclosed in the official gazette of National UnexaminedPatent Publication 2,000-500,796 and HEI-11-503,177 are dehydrated,therefore, they cannot be fully satisfactorily dehydrated or, whencompelled to be sufficiently dehydrated, they possibly encounterfracture during the course of dehydration. The porous cross-linkedpolymer obtained by polymerizing an HIPE, though depending on the W/Oratio of the HIPE to be used, has a water content reaching a level inthe range of 300-25,000 (w/w)% based on the mass of the polymer and,therefore, is particularly deficient in mechanical strength. Once thecompressing operation of a metal roll inflicts a crack on the surface ofthe product for contact with the roll, this crack will form a cause forlowering the rating of the product in appearance. Further, when theproduct happens to sustain a crack or a bend, for example, this damagewill form a cause for degrading the water absorbing property which theproduct is inherently required to possess and will possibly inducedegradation of the quality of the product in terms of function.

Further, for the purpose of allowing a salt capable of stabilizingemulsification during the formation of an HIPE to be incorporated in thewater phase thereby enabling the emulsification to proceed smoothly,such an electrolyte as calcium chloride is added to the emulsion. Whenthe dehydration resorts solely to the use of a porous nip roll furnishedwith an aspiration mechanism, therefore, the roll is suffered to gatherdirt on the surface thereof and the pores in the roll are clogged withdirt possibly to the extent of degrading the capacity of dehydration andinterrupting the operation of dehydration because the sparingly solubleprecipitate and the unaltered monomer are also subjected to dehydration.This interruption of the dehydrating operation forms a cause forinducing degradation of the efficiency of production particularly whenthe porous cross-linked polymer is continuously produced at a highspeed.

An object of this invention, therefore, is to provide a method forcausing a porous cross-linked polymer obtained by polymerizing an HIPEto be dehydrated without inflicting any damage on the polymer and amethod for producing a porous cross-linked polymer sheet allowing adehydrating treatment to be performed continuously and smoothly thereon.

DISCLOSURE OF THE INVENTION

This invention has been perfected based on the discovery that when aporous cross-linked polymer obtained by polymerizing an HIPE isdehydrated by a treatment using to a non-woven fabric roll and anaspiration mechanism in combination, a polymer sheet free from suchdamage as fold and crack can be produced with very high efficiency.

Specifically, this invention is accomplished by the following item (1).

(1) A method for the production of a porous cross-linked polymer sheet,comprising the steps of forming and polymerizing an HIPE therebyobtaining a porous cross-linked polymer and dehydrating the porouscross-linked polymer by using a non-woven fabric roll furnished with anaspiration mechanism.

In accordance with this invention, the use of non-woven fabric rollsfurnished with an aspiration mechanism enables a porous cross-linkedpolymer to be dehydrated to an extremely low water content. In order forthe conventional method to acquire this low water content, it generallynecessitates an increase in the linear pressure of roll and consequentlyinduces the porous cross-linked polymer to sustain fracture. In thisinvention, however, by particularly specifying the thickness of theporous cross-linked polymer, it is made possible to dehydrate thepolymer sheet thoroughly to the center thereof with unusually highefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the relation between the water contentand the volume of the porous cross-linked polymer when the porouscross-linked polymer is dehydrated.

FIG. 2 is a schematic side view illustrating a typical mode ofembodiment of an appropriate continuous polymerization device in amethod for the production of c porous cross-linked polymer contemplatedby this invention.

FIG. 3 is a schematic diagram illustrating the production of a non-wovenfabric roll furnished with an aspiration mechanism by the use of discscut from a non-woven fabric.

FIG. 4 is a diagram illustrating a method for using an endless meshbelt.

1—Roll proper, 2—porous, 3—flange, 4—non-woven fabric in the shape of adisc, 5—endless mesh belt, 101—HIPE, 102—Porous cross-linked polymer,119—RIPE supplying device, 201—endless belt type conveyor (furnishedwith drive conveying device), 203, 205—sheet material, 207,208—unwinding roller, 209, 211—roller, 212, 213—rewinding roller,215—polymerization furnace, 217—heating means, 219—heating means (hotwater shower), 301—non-woven fabric roll furnished with aspirationmechanism, 302—transporting conveyor, 303—dehydrating device, 402—bandknife, 401—slicer, 403, 405—-transporting conveyor

BEST MODE OF EMBODYING THE INVENTION

This invention primarily concerns a method for the production of aporous cross-linked polymer sheet, comprising the steps of forming andpolymerizing a water-in-oil type high internal phase emulsion therebyobtaining a porous cross-linked polymer and dehydrating the porouscross-linked polymer by using a non-woven fabric roll furnished with anaspiration mechanism.

While the dehydration contemplated by this invention is characterized byusing a non-woven fabric roll furnished with an aspiration mechanism,particularly by forming a coat of non-woven fabric on the surface of thenip rolls, it is made possible to attain instantaneous removal of thewater expelled by compression. Especially, the porous cross-linkedpolymer itself is friable because it contains a large volume of water.When the linear pressure of the rolls is heightened for the sake ofdehydration, therefore, the polymer possibly sustains a crack orcollapses readily. In the present invention, however, by using anon-woven fabric on the surface of the nip rolls, it is made possible tosecure fully satisfactory initial absorbing power which enables theporous cross-linked polymer to be very efficiently dehydrated withoutinfliction of any fracture.

Particularly, as respects the mechanism for dehydrating a porouscross-linked polymer, the squeezing, when considered physically, iscapable of discharging the water from the polymer in an amountproportionate to the decrease in the volume caused by the linearpressure of the nip rolls during the initial stage thereof. Since thewater which remains in the porous of the polymer relies on thecapillarity to support the decrease in the volume caused by the linearpressure of the rolls, this decrease in the volume persists as permanentcompression strain in the porous cross-linked polymer. This inventionrefers to this phase of squeezing as the first stage. As the squeezingadvances, however, the elastic force of the porous cross-linked polymergrows to the extent of surpassing the permanent compression strain dueto the capillarity. In this case, the polymer is caused to emit water bythe compression of the nip rolls. When the polymer is relieved of thecompression, it immediately expands and instantaneously retrieves theemitted water. This invention refers to this phase of squeezing as thesecond stage. This process of squeezing will be described below withreference to FIG. 1. FIG. 1 is a diagram illustrating the relationbetween the change in the water content due to the dehydration and thechange in the volume of the sheet observed when a porous cross-linkedpolymer sheet having a water content of 4,500 (w/w)% and a thickness of10 mm is dehydrated with the nip rolls. During the decrease of the watercontent from 4,500 (w/w)% to 1,000 (w/w)%, the volume decreases inaccordance as the dehydration by the compression advances. This formsthe first stage. As the water content falls below 1,000 (w/w)%, however,the water content is changed but the volume of the sheet is not changed.This process of the squeezing constitutes itself the second stage. Ithas been demonstrated that this invention, owing to the use of non-wovenfabric rolls, enables the ratio of dehydration particularly in thesecond stage to be efficiently exalted and the relevant water content tobe lowered to the extreme bottom magnitude.

Now, this invention will be described in detail below.

[I] Dehydrating Treatment

(1) Non-Woven Fabric Roll Furnished with an Aspiration Mechanism

The non-woven fabric rolls furnished with an aspiration mechanism andused in this invention comprise nip rolls which are liquid absorbingparts and a coat of non-woven fabric covering the surfaces of the niprolls. They do not need to be particularly discriminated but are onlyrequired to be possessed of the construction described above.

(a) Non-Woven Fabric

The term “non-woven fabric” as used in this invention refers to asheet-like structure of single fibers obtained not by spinning, weaving,or knitting fibers but by accumulating fibers and joining orintertwining them by means of a thermal, mechanical, or chemical action.

The fibers for forming the non-woven fabric contemplated by thisinvention do not need to be particularly discriminated. Syntheticfibers, plant fibers, animal fibers, glass fibers, metallic fibers,carbon fibers, ceramic fibers, and mixtures thereof can beadvantageously used for the non-woven fabric. The synthetic fibersinclude polyamide fibers, polyester fibers, polyacrylonitrile fibers,polyvinyl alcohol fibers, polyvinyl chloride fibers, polyvinylidenechloride fibers, polyolefin fibers, Alamides fibers, and polyurethanefibers, polyethylene fibers, polypropylene fibers, polyfluoroethylenefibers, acetyl cellulose fibers, and rayon fibers, for example. Theanimal fibers include silk, wool, goat's hair, mohair, cashmere, alpaca,camel's hair, rabbit's hair, beaver's hair, and pig's hair, for example.The plant fibers include skin fibers of cotton, flax, and hemp and veinfibers of hemp, for example. The glass fibers are fibers extruded frommolten glass and they come in two types, i.e. short fibers and longfibers.

The metallic fibers include stainless steel fibers and aluminum fibers,for example.

The ceramic fibers include silica fibers and alumina fibers, forexample.

For this invention, it is preferable to use synthetic fibers and morepreferable to use polyester fibers particularly from the viewpoint ofthermal resistance, resistance to acids and alkalis, and durability.This invention allows these species of fibers to be used either singlyor in the form of a mixture of two or more species.

The diameter of the fibers forming the non-woven fabric is properly notmore than 50 deniers, preferably in the range of 0.02-10 deniers, andparticularly preferably in the range of 0.02-0.4 deniers. The reason forthe preferability of using such extremely thin fibers is that, duringthe dehydrating treatment, the initial liquid absorbing power due to thecapillarity and the ability to retain the liquid absorbing power due tothe transfer of the absorbed liquid can be simultaneously secured. Ifthe size of fiber is unduly small, the fibers will be liable to betraydeficiency in liquid retaining power. Conversely, if the size exceeds 50deniers, the excess will be at a disadvantage in decreasing the emptyspace for admitting the absorbed liquid and quickly losing the liquidabsorbing power.

The non-woven fabric can be manufactured by any of the known methods. Itis obtained, for example, by preparatorily forming layers of fibersheets and subsequently binding these layers of fiber sheets. The drymethod, spun bond method, melt blow method, and wet method, for example,are available for the formation of the layer of fiber sheets. Thethermal bond method, chemical bond method (impregnating method), needlepunch method, and water entanglement method, for example, are availablefor binding the layer of fiber sheets. This invention is allowed to useany of the methods mentioned above for forming the layer of fiber sheetand use any of the methods mentioned above for bonding the layer offiber sheets.

The liquid absorbing power of the non-woven fabric is properly not lessthan 1.0, preferably in the range of 1.0-15, more preferably in therange of 1.0-5.0, and particularly preferably in the range of 1.3-4.0.If the liquid absorbing power falls short of 1.0, the shortage will beat a disadvantage in lowering the initial liquid absorbing power.Conversely, if the liquid absorbing power is unduly large, the excesswill be at a disadvantage in effecting dehydration, aspiration, andreabsorption insufficiently.

The “liquid absorbing power” defined by this invention is to beexpressed by the ratio of increase of the mass which is determined by anexperiment which comprises keeping a given non-woven fabric, 30 cm×30 cmin area, immersed in water for one hour, then squeezing the wet fabricwith a mangle, performing these steps up to four repetitions, thenimmersing the sample fiber in water and removing it from the water,leaving the wet sample fiber on a filter paper for 3 seconds and, at thesame time, allowing the filter paper to absorb the water from thesurface of the fiber, and immediately determining the mass of the samplefiber. The liquid absorbing power, therefore, assumes the numericalvalue which is expressed as (wet mass−dry mass)/dry mass. The non-wovenfabric to be used in this invention does not need to be particularlydiscriminated in such factors as thickness but is only required topossess the liquid absorbing power defined above.

The non-woven fabric of this invention may have a macromolecular elasticsubstance fill the gaps in the non-woven fabric. The reason for thepresence of this macromolecular elastic substance is that it enables thewhole non-woven fabric to be retained at a moderate elasticity, improvesthe fabric in the liquid absorbing power and the ability to retain theabsorbed liquid, and stabilizes the fabric in shape.

This macromolecular elastic substance does not need to be discriminatedon account of the kind of substance and the amount of the substance tobe incorporated but is only required to possess liquid absorbing powerof not less than 1.0, it is preferred to be polyurethane elastomer. Thefiber sheet which has the macromolecular elastic substance fill the gapsin the non-woven fabric thereof can be easily obtained by any of theknown methods. Specifically, the non-woven fabric containing themacromolecular elastic substance in the manner described above can bemanufactured by preparing a non-woven fabric, then impregnating orcoating the non-woven fabric with a solution or dispersion of apolyurethane elastomer, and finally allowing the immersed or coatednon-woven fabric to undergo wet coagulation.

This invention particularly prefers a porous structure in which thevoids in a non-woven fabric formed by three-dimensionally intertwiningextremely thin fibers are filled with a macromolecular elasticsubstance. The fiber sheet of this porous structure is obtained byforming a non-woven fabric of extremely thin fibers by such a method asmixed spinning method, direct spinning method, or composite spinningmethod and subsequently performing the steps of decrease of diameter,impregnation with a macromolecular elastic substance, and wetcoagulation in a proper order.

(b) Aspiration Mechanism

The non-woven fabric rolls to be used in this invention have the niproll parts thereof serve as liquid absorbing parts and have the liquidabsorbing parts furnished with an aspiration mechanism. The liquidabsorbing parts, therefore, are shaped like rolls. The aspirationmechanism does not need to be particularly discriminated but is onlyrequired to be capable of positively aspirating liquid from thenon-woven fabric. The nip rolls, for example, may be provided on theinner parts or the surface parts thereof with an aspiration mechanismconnected to a negative pressure generating source. When the aspirationmechanism is installed inside, it suffices to decompress the interiorsof the cylindrical shafts of the rolls.

(c) Method for Production of Non-Woven Fabric Roll

The non-woven fabric roll furnished with the aspiration mechanism andused in this invention can be produced, for example, by rolling ametallic sheet made of stainless steel and possessed of numerous throughholes into a roll itself and wrapping the non-woven fabric mentionedabove around the surface of the roll itself. For the purpose ofuniformizing the absorption property in this case, it is permissible toapply an adhesive agent in the form of disperse dots to the surface or,when the non-woven fabric happens to contain a thermally fusible resin,in the form of a continuous layer on the surface by means ofthermo-compression bonding or even by sewing a non-woven fabric to thesurface. The non-woven fabric roll to be used in this invention imposesno limit on the number of layers of non-woven fabric to be wrappedaround the nip roll. Nevertheless, the number of such layers is properlyin the range of 1-15, preferably in the range of 1-5.

By a procedure which comprises forming a flange 3 in one terminal partof a roll itself 1 possessed of minute holes 2, inserting a plurality ofnon-woven fabrics 4 cut in the shape of a disc of necessary sizesuccessively around the flange 3 from the other terminal of the rollproper 1 inward, and then forming a flange in the other terminal part asillustrated in FIG. 3, it is made possible to produce a non-woven fabricroll possessed of a liquid absorbing mechanism which is composed of thetwo flanges and the plurality of non-woven fabric sheets. Generally, onthe plurality of discs superposed in the manner described above, apressing work is performed in the direction of superposition. Therebydense and compact rolls are obtained. When the porous cross-linkedpolymer sheet is dehydrated by using a plurality of such non-wovenfabric rolls, the nip rolls are allowed to use non-woven fabrics variedin kind.

When the rolls for forming the liquid absorbing parts are manufacturedby superposing a plurality of discs of non-woven fabric, they are at anadvantage in not only excelling in initial liquid absorbing power andability to retain the absorbed liquid but also avoiding infliction ofdamage to the surface of the object under treatment. Further, they arecapable of uniformly forming the roll surface because of the unnecessityfor stopping the terminal parts of the non-woven fabric with an adhesiveagent. Moreover, they are enabled to acquire easily an increased widthand enjoy stability of shape by proportionately increasing the number ofdiscs to be superposed and, even when the roll surface happens tosustain damage due to long use, they are enabled to acquire again a fineroll surface by saving the surface.

Even when the non-woven fabric uses extremely thin fibers, it is enabledto form very thin continuous voids so long as it is filled with a porousstructure of macromolecular elastic substance. These voids give rise tothe so-called phenomenon of capillarity and impart an excellent liquidabsorbing power to the liquid filtering part of the roll. When the discsof the non-woven fabric are superposed to form a roll, the effect of thecapillarity can be manifested to the maximum because no interrupted spotoccurs in the extremely thin continuous voids in the direction ofthickness of the roll.

When the non-woven fabric of the roll is composed of extremely thinfibers and a macromolecular elastic substance, the roll surface is softand dense and capable of squeezing the article under treatment withoutappreciably exerting any linear pressure thereon and uniformly squeezingthe object without sustaining any damage. Particularly, the fibers whichare very thin are at an advantage in enabling the effect underdiscussion to be manifested conspicuously. When the roll formed bysuperposing a plurality of discs of non-woven fabric is used as a coreand a sheet of a fine liquid absorbing property is wrapped around thesurface of this roll, since the product consequently obtained ispossessed of an outer layer part formed of the sheet, the core isprotected from such troubles as developing blockage and decreasing thediameter and undulating the surface owing to a damage inflicted by theobject under treatment. Thus, the core is enabled to retain theexcellent liquid absorbing property over a long period of time and keepthe shape of the core intact meanwhile.

(2) Method for Dehydration

Now, one example of the method for the continuous process of producingthe porous cross-linked polymer sheet of this invention will bedescribed below with reference to FIG. 2. First, as illustrated in FIG.2, an HIPE 101 is continuously supplied from an HIPE supplying device119 onto a sheet material 203 to form a sheet of a prescribed thicknessby adjusting the set height of a roller 209. The rotating speeds ofunwinding and rewinding rollers 208, 212 are so controlled that thesheet material 203 may be synchronized with a conveyor belt 201. A sheetmaterial 205, while exerting tension necessary for fixing the thicknessof the HIPE 101, allows the rotating speed of itself to be controlled bythe rollers 209, 211 and unwinding and rewinding rollers 207, 213.Inside a polymerization furnace 215, the HIPE 101 is polymerized by aheating means 219 formed of a hot water shower and disposed below theconveyor belt 201 and a heating means 217 formed of a hot aircirculating device and disposed above the conveyor belt 201 so as toobtain a porous cross-linked polymer 102. The polymer from which theupper and lower sheet materials 203, 205 have been separated is mountedon the belt which is being revolved by a conveyor 302 in motion on therolls of a dehydrating device 303, nipped between non-woven fabric rolls301 disposed on and under the belt and furnished with an aspirationmechanism, and dehydrated by revolving the rolls. Optionally, thedehydrated porous cross-linked polymer 102 may be transferred to anendless band knife type slicer 401 and sliced in the direction ofthickness by a band knife 402 in motion.

This invention does not need to limit the production of the porouscross-linked polymer sheet to the continuous method described above.Optionally, the porous cross-linked polymer sheet may be produced bypolymerizing an HIPE batchwise in the form of a sheet and thendehydrating the sheet of the polymer or by polymerizing an HIPEbatchwise and slicing the produced sheet of polymer into pieces of aproper thickness and dehydrating these slices either continuously orbatchwise.

(a) Sheet Transferring Speed

The dehydration of the porous cross-linked polymer contemplated by thisinvention is attained by using non-woven fabric rolls furnished with anaspiration mechanism as nip rolls in the known step of dehydrationresorting to nip rolls and subjecting the porous cross-linked polymer tothe action of the non-woven fabric rolls. Generally, the purpose ofcontinuously dehydrating a porous cross-linked polymer sheet isattained, for example, by continuously supplying an HIPE onto the beltof a belt conveyor in motion which is so constructed as to heat the beltsurface with a heating device, forming and polymerizing the HIPE in theform of a smooth sheet on the belt, and subsequently dehydrating theproduced sheet of polymer with the nip rolls mentioned above.

During the dehydration by the non-woven fabric rolls mentioned aboveaccording to this invention, the transferring speed is properly in therange of 0.5-150 m/min, preferably in the range of 2-100 m/min., andparticularly preferably in the range of 5-75 m/min. Though the linearpressure of the roll during this dehydration is properly adjusted, it ispreferred to be not more than 30 kg/cm. The linear pressures exerted bythe individual non-woven fabric rolls used in the same line may beidentical or different. If the linear pressure of a roll is unduly low,the shortage will be at a disadvantage in preventing the dehydration byphysical compression from being effected fully satisfactorily.Conversely, if the linear pressure of a roll exceeds 30 kg/cm, theexcess will be at a disadvantage in inflicting damage to the porouscross-linked polymer without reference to the sheet transferring speed.If the sheet transferring speed falls short of 0.5 m/min., the shortagewill result directly in degrading the productivity. Conversely, if thisspeed exceeds 150 m/min., the excess will be at a disadvantage inlowering the efficiency of dehydration and necessitating an addition tothe length of the dehydrating device.

The mechanical strength of the porous cross-linked polymer is varied bythe W/O ratio of the HIPE. This invention does not need to limit the W/Oratio particularly. Properly, the W/O is not less than 3/1. It ispreferably in the range of 10/1-250/1, and particularly preferably inthe range of 10/1-80/1. If the W/O ratio falls short of 3/1, theshortage will induce an excessive increase in the content of the oilphase containing the polymerizing monomer and, in spite of animprovement in mechanical strength, preclude the formation of a porouscross-linked polymer sheet excelling in water retaining power per unitvolume. Conversely, if the W/O ratio exceeds 250/1, the excess willpossibly inflict a crack in the sheet during the course of dehydrationor even compel the sheet to break asunder.

(b) Mesh Belt

This invention permits the porous cross-linked polymer to be dehydratedthrough the medium of a mesh belt which is disposed each on the upperand lower sides or only on the lower side of the porous cross-linkedpolymer interposed between the opposed non-woven fabric rolls. When theporous cross-linked polymer is dehydrated with the non-woven fabricrolls, the parts thereof which are nipped by the non-woven fabric rollsare liable to break and cleave because the linear pressure of roll isexerted on a narrow range of the porous cross-linked polymer. Theintervention of the mesh belt, however, facilitates the conveyance ofthe porous cross-linked polymer having low strength and a high watercontent, alleviates the shear force exerted by the roll and the linearpressure of roll, and prevents the porous cross-linked polymer fromcrushing.

The mesh belt described above is produced by forming a mesh of metallicwires, synthetic resin, plant fibers, animal fibers, glass fibers, and amixture thereof. The raw materials used for the metallic wires formingthe mesh belt include stainless steel, aluminum, iron, and zinc, forexample. The synthetic fibers usable for the mesh belt include polyamideresin, polyester resin, polyacrylonitrile resin, polyvinyl alcoholresin, polyvinyl chloride resin, polyvinylidene chloride resin,polyurethane resin, polyethylene resin, polypropylene resin,polyfluoroethylene resin, acetyl cellulose resin, and rayon resin, forexample. The animal fibers usable for the mesh belt include silk, wool,goat's hair, mohair, cashmere, alpaca, camel's hair, rabbit's hair,beaver's hair, and pig's hair, for example. The plant fibers includeskin fibers of cotton, flax, and hemp and vein fibers of hemp, forexample. The glass fibers are fibers extruded from molten glass and theycome in two types, i.e. short fibers and long fibers. For thisinvention, it is preferable to use synthetic fibers and more preferableto use polyester fibers particularly from the viewpoint of thermalresistance, resistance to acids and alkalis, and durability. Thisinvention allows these species of fibers to be used either singly or inthe form of a mixture of two or more species.

The mesh interval in the mesh belt is required to have air permeability,which is preferably not less than 5000 ml/cm²·min and particularlypreferably in the range of 10000-60000 ml/cm²·min. If the mesh intervalfalls short of 5000 ml/cm²·min, the shortage will bring insufficientliquid passage.

The width of the mesh belt may be equal to or more than the width of thesheet of porous cross-linked polymer. The mesh belt of this constructioncan be mounted on the porous cross-linked polymer at the time ofcompletion of the polymerization of an HIPE. After the completion of thedehydration, it can be recovered for reuse by being rolled up.Preferably, endless mesh belts are disposed respectively on the upperand lower dehydrating rolls. The case of effecting the dehydration bythe use of such endless mesh belts will be described below withreference to FIG. 4. To begin with, FIG. 4 illustrates an example ofusing the mesh belts in part of the step of dehydration depicted in FIG.2. When a porous cross-linked polymer 102 is to be dehydrated withnon-woven fabric rolls 301 furnished with an aspiration mechanism, thedehydration is effected with the aid of an endless belt which is passedaround a plurality of rolls 301 disposed on and beneath the porouscross-linked polymer 102 and rolls 211 as illustrated in FIG. 4. Theendless belt does not need to be used on all the non-woven fabric rolls301 which are furnished with an aspiration mechanism. It may be used onpart of the non-woven fabrics 301 which are possessed of an aspirationmechanism as illustrated in FIG. 4B.

(c) Two-Stage Dehydration

This invention does not impose any restriction on the number ofnon-woven fabric rolls to be used. In this case, the nip rolls to beused for the dehydrating treatment contemplated by this invention do notneed to be invariably non-woven fabric rolls furnished with anaspiration mechanism. Optionally, the porous cross-linked polymerobtained by polymerizing an HIPE may be preparatorily dehydrated withsome other nip rolls and subsequently dehydrated by using such non-wovenfabric rolls are possessed of an aspiration mechanism. Properly, theratio of dehydration after the preliminary squeezing keeps not more than1000 (w/w)% where the initial water content is in the range of 1000-3000(w/w)%, falls in the range of 500-2000 (w/w)% where the initial watercontent is in the range of 3000-7000 (w/w)%, and falls in the range of1000-5000 (w/w)% where the initial water content is in the range of7000-25000 (w/w)%.

As described previously with reference to FIG. 1, when the dehydrationis implemented by the use of nip rolls, the mechanism for dehydrationmay be divided from the viewpoint of the water content into the firststage for generating permanent compression strain and the subsequentsecond stage. Since this invention produces an unusually fine effectparticularly in the second stage of dehydration, the use in the secondstage of non-woven fabric rolls furnished with an aspiration mechanismpermits use of other nip rolls particularly in the first stage. Thereason for the selective use only in the second stage is that itproduces an extremely small effect on the final ratio of dehydration,namely on the ratio of eventual dehydration. In this respect, when theratio of dehydration in the preliminary squeezing is fixed as anumerical value variable with the initial water content of the porouscross-linked polymer within the range mentioned above, the lower limitof the ratio of dehydration at the initial water content corresponds tothe state of the first stage in the dehydration mechanism mentionedabove. The water discharged from the porous cross-linked polymercontains the impurities occurring during the preparation of an HIPE andthe unaltered monomer, salt, and un-crosslinked polymer. Thus, when thenon-woven fabric rolls furnished with an aspiration mechanism areimmediately put to use, these defiling substances adhere to thenon-woven fabric and form a cause for degrading the ability ofabsorption, obliging the apparatus to make early stop or receiveperiodic inspection, and lower the efficiency of production of theporous cross-linked polymer.

Specifically, when the two-stage squeezing is implemented in the methodfor continuous production of a porous cross-linked polymer sheet, anoperation which comprises disposing a plurality of devices ofdehydrating non-woven fabric rolls 301 shown in FIG. 2, carrying out thepreliminary squeezing by the first dehydrating device 303, andsubsequently performing the dehydration by the use of the non-wovenfabric rolls furnished with an aspiration mechanism in the seconddehydrating device 303 may be adopted. Incidentally, the two-stagedehydration does not need to be limited to the continuous methoddescribed above. Optionally, the porous cross-linked polymer sheetobtained batchwise is subjected not continuously to the dehydration withthe non-woven fabric rolls furnished with a preliminary squeezingmechanism and an aspiration mechanism.

The nip rolls which fit the preliminary squeezing may be selected fromamong a rich variety of known nip rolls. They include rubber rolls,metal rolls, nip rolls formed by coating metal rolls with a felt sheet,and non-woven fabric rolls provided with an aspiration mechanism.

The sheet transferring speed used in the preliminary squeezing,similarly to that in the formal squeezing is properly in the range of0.5-150 m/min., preferably in the range of 2-100 m/min., andparticularly preferably in the range of 5-75 m/min. The linear pressureof roll in this case, though properly adjusted, is preferred to be notmore than 30 kg/cm. The linear pressures by the individual non-wovenfabric rolls may be identical or different. If the linear pressure of aroll is unduly low, the shortage will be at a disadvantage in preventingthe dehydration by physical compression from being effected fullysatisfactorily. Conversely, if the linear pressure of a roll exceeds 30kg/cm, the excess will be at a disadvantage in inflicting damage to theporous cross-linked polymer without reference to the sheet transferringspeed. If the sheet transferring speed falls short of 0.5 m/min., theshortage will result directly in degrading the productivity. Conversely,if this speed exceeds 150 m/min., the excess will be at a disadvantagein lowering the efficiency of dehydration and necessitating an additionto the length of the dehydrating device.

When a plurality of nip rolls are used in the preliminary squeezing, thelinear pressures of these rolls maybe identical or different. As regardsthe mesh belt mentioned above, it may be used during the course of thepreliminary squeezing or the dehydration may omit using the mesh belt.

[II] Preparation of Porous Cross-Linked Polymer

Now, the porous cross-linked polymer to be dehydrated in accordance withthis invention will be described below. The porous cross-linked polymerwhich is subjected to the dehydration contemplated by this invention isobtained by preparing an HIPE and then polymerizing the HIPE. Now, thepreparation of the porous cross-linked polymer will be described below.

(1) Raw Materials Used for HIPE

The raw materials to be used for an HIPE are only required to contain(a) a polymerizing monomer, (b) a cross-linking monomer, and (c) asurfactant, as an essential component for forming an oil phase and (d)water as an essential component for forming a water phase. They mayfurther contain, when necessary, (e) a polymerization initiator, (f) asalt, and (g) other additives as arbitrary component for forming the oilphase and/or the water phase.

(a) Polymerizing Monomer

The monomer composition essential for the composition of the HIPEmentioned above is a polymerizing monomer possessing one polymerizingunsaturated group in the molecule thereof. Though it does not need to beparticularly discriminated but has only to be capable of beingpolymerized in a dispersion or a water-in-oil type high internal phaseemulsion and allowed to form an emulsion consequently. It preferablycontains a (meth)acrylic ester at least partly, more, preferablycontains not less than 20 mass % of the (meth)acrylic ester, andparticularly preferably contains not less than 35 mass % of the(meth)acrylic ester. When the (meth)acrylic ester is contained as apolymerizing monomer possessing one polymerizing unsaturated group inthe molecule thereof proves advantageous because the produced porouscross-linked polymer abounds in flexibility and toughness.

As concrete examples of the polymerizable monomer which is usedeffectively in this invention, allylene monomers such as styrene;monoalkylene allylene monomers such as ethyl styrene, α-methyl styrene,vinyl toluene, and vinyl ethyl benzene; (meth) acrylic esters such asmethyl (meth) acrylate, ethyl (meth)acrylate, butyl (meth)acrylate,isobutyl (meth)acrylate, isodecyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,cyclohexyl (meth)acrylate, and benzyl (meth)acrylate;chlorine-containing monomers such as vinyl chloride, vinylidenechloride, and chloromethyl styrene; acrylonitrile compounds such asacrylonitrile and methacrylonitrile; and vinyl acetate, vinylpropionate, N-octadecyl acrylamide, ethylene, propylene, and butene maybe cited. These polymerizable monomers may be used either singly or inthe form of a combination of two or more members.

The content of the polymerizing monomer is preferred to be in the rangeof 10-99.9 mass %, based on the total mass of the monomer compositionconsisting of the polymerizing monomer and a cross-linking monomer. Thereason for this range is that the produced porous cross-lined polymer isenabled to acquire pores of minute diameters. The range is morepreferably 30-99 mass % and particularly preferably 30-70 mass %. If thecontent of the polymerizing monomer is less than 10 mass %, the producedporous cross-linked polymer will be possibly friable and deficient inwater absorption ratio. Conversely, if the content of the polymerizingmonomer exceeds 99.9 mass %, the porous cross-linked polymerconsequently produced will be possibly deficient in strength and elasticrecovery power and incapable of securing sufficient amount of waterabsorbed and sufficient velocity of water absorption.

(b) Cross-Linking Monomer

The other monomer composition essential for the composition of the HIPEmentioned above is a cross-linking monomer possessing at least twopolymerizing unsaturated groups in the molecule thereof. Similarly tothe polymerizing monomer mentioned above, it does not need to beparticularly discriminated but has only to be capable of beingpolymerized in a dispersion or a water-in-oil type high internal phaseemulsion and allowed to form an emulsion consequently.

As concrete examples of the cross-linking monomer which is effectivelyusable herein, aromatic monomers such as divinyl benzene, trivinylbenzene, divinyl toluene, divinyl xylene, divinyl naphthalene, divinylalkyl benzenes, divinyl phenanthrene, divinyl biphenyl, divinyl diphenylmethane, divinyl benzyl, divinyl phenyl ether, and divinyl diphenylsulfide; oxygen-containing monomers such as divinyl furan;sulfur-containing monomers such as divinyl sulfide and divinyl sulfone;aliphatic monomers such as butadiene, isoprene, and pentadiene; andesters of polyhydric alcohols with acrylic acid or methacrylic acid suchas ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate,triethylene glycol di(meth)acrylate, polyethylene glycoldi(meth)acrylate, 1,3-butane diol di(meth)acrylate, 1,4-butane dioldi(meth)acrylate, 1,6-hexane diol di(meth)acrylate, octane dioldi(meth)acrylate, decane diol di(meth)acrylate, trimethylol propanedi(meth)acrylate, trimethylol propane tri(meth)acrylate, pentaerythritoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol di(meth)acrylate,dipentaerythritol tri(meth)acrylate, dipentaerythritoltetra(meth)acrylate, N,N′-methylene bis (meth)acryl amide, triallylisocyanurate, triallyl amine, tetraallyloxy ethane, hydroquinone,catechol, resorcinol, and sorbitol may be cited. These cross-linkingmonomers may be used either singly or in the form of a combination oftwo or more members.

The content of the cross-linked monomer is properly in the range of0.1-90 mass %, preferably 1-70 mass %, and particularly preferably 30-70mass %, based on the total mass of the monomer composition consisting ofthe polymerizing monomer mentioned above and the cross-linking monomermentioned above. If the content of the cross-linked monomer is less than0.1 mass %, the produced porous cross-linked polymer will possibly bedeficient in strength and elastic recovery force, unable to effectabsorption sufficiently per unit volume or unit mass, and incapable ofsecuring absorption in a sufficient amount at a sufficient velocity.Conversely, if the content of the cross-linked monomer exceeds 90 mass%, the porous cross-linked polymer produced consequently will possiblybe friable and deficient in water absorption ratio.

(c) Surfactant

The surfactant which is essential for the composition of the HIPEmentioned above does not need to be particularly discriminated but hasonly to be capable of emulsify a water phase in an oil phase forming theHIPE. It is not limited to the specific examples cited above but may beselected from the nonionic surfactants, cationic surfactants, anionicsurfactants and ampholytic surfactants heretofore known to the art.

Among these surfactants, as concrete examples of the nonionicsurfactant, nonyl phenol polyethylene oxide adduct; block polymer ofethylene oxide and propylene oxide; sorbitan fatty acid esters such assorbitan monolaurate, sorbitan monomyristylate, sorbitan monopalmitate,sorbitan monostearate, sorbitan tristearate, sorbitan monooleate,sorbitan trioleate, sorbitan sesquioleate, and sorbitan distearate;glycerin fatty acid esters such as glycerol monostearate, glycerolmonooleate, diglycerol monooleate, and self-emulsifying glycerolmonostearate; polyoxyethylene alkyl ethers such as polyoxyethylenelauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearylether, polyoxyethylene oleyl ether, and polyoxyethylene higher alcoholethers; polyoxyethylene alkylaryl ethers such as polyoxyethylene nonylphenyl ether; polyoxyethylene sorbitan fatty acid esters such aspolyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitanmonomyristylate, polyoxyethylene sorbitan monopalmitate, polyoxyethylenesorbitanmonostearate, polyoxyethylene sorbitan tristearate,polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitantrioleate; polyoxyethylene sorbitol fatty acid esters such as tetraoleicacid polyoxyethylene sorbit; polyoxyethylene fatty acid esters such aspolyethylene glycol monolaurate, polyethylene glycol monostearate,polyethylene glycol distearate, and polyethylene glycol monooleate;polyoxyethylene alkyl amines; hydrogenated polyoxyethylene castor oil;and alkyl alkanol amides may be cited. These nonionic surfactants havingHLB values of not more than 10, more preferably in the range of 2-6,prove preferable. It is permissible to use two or more such nonionicsurfactants in combination. The combined use possibly results instabilizing the HIPE.

As concrete examples of the cationic surfactant, quaternary ammoniumsalts such as stearyl trimethyl ammonium chloride, ditallow dimethylammonium methyl sulfate, cetyl trimethyl ammonium chloride, distearyldimethyl ammonium chloride, and alkylbenzyl dimethyl ammonium chloride;alkyl amine salts such as coconut amine acetate and stearyl amineacetate; alkyl betaines such as lauryl trimethyl ammonium chloride,lauryl betaine, stearyl betaine, and lauryl carboxymethylhydroxyethylimidazolinium betaine; and amine oxides such as lauryldimethyl amine oxide may be cited. The use of the cationic surfactantcan impart excellent antibacterial properties to the porous cross-linkedpolymer when the polymer is used for an absorbent material, for example.

The anionic surfactant of a kind possessing an anionic moiety and anoil-soluble moiety can be advantageously used. As concrete examples ofanionic surfactant, alkyl sulfates such as sodium dodecyl sulfate,potassium dodecyl sulfate, and ammonium alkyl sulfate; sodium dodecylpolyglycol ether sulfate; sodium sulforicinoate; alkyl sulfonates suchas sulfonated paraffin salts; alkyl sulfonates such as sodium dodecylbenzene sulfonate, alkali metal sulfates of alkali phenolhydroxyethylene; higher alkyl naphthalene sulfonates; fatty acid saltssuch as naphthalene sulfonic acid formalin condensate, sodium laureate,triethanol amine oleate, and triethanol amine apiate; polyoxyalkyl ethersulfuric esters; sulfuric esters of polyoxyethylene carboxylic ester andpolyoxyethylene phenyl ether sulfuric esters; succinic acid dialkylester sulfonates; and reactive anion emulsifiers possessed of a doublebond such as polyoxy ethylene alkyl aryl sulfates may be cited. An HIPEmay be prepared by using an anionic surfactant in combination with acationic surfactant.

The combined use of the nonionic surfactant and the cationic surfactantmay possibly improve the HIPE in stability.

The content of the surfactant mentioned above is properly in the rangeof 1-30 mass parts, preferably 3-15 mass parts, based on 100 mass partsof the total mass of the monomer composition consisting of thepolymerizing monomer and the cross-linked monomer. If the content of thesurfactant is less than 1 mass part, the shortage will possibly depriveof the HIPE of stability of dispersion and prevent the surfactant frommanifesting the effect inherent therein sufficiently. Conversely, if thecontent of the surfactant exceeds 30 mass parts, the excess willpossibly render the produced porous cross-linked polymer unduly friableand fail to bring a proportionate addition to the effect thereof and doany good economically.

(d) Water

The water essential for the composition of the HIPE mentioned above maybe city water, purified water or deionized water. Alternatively, with aview to utilizing to advantage the waste water resulting from theproduction of the porous cross-linked polymer, this waste water may beadopted in its unmodified form or after undergoing a prescribedtreatment.

The content of the water may be suitable selected, depending on the kindof use (such as, for example, a water absorbent material, an oilabsorbent material, sound insulation material, or filter) for which theporous cross-linked polymer possessing continuous cells is intended.Since the hole ratio of the porous cross-linked polymer material isdecided by varying the water phase/oil phase (W/O) ratio of the HIPE,the amount of water to be used is automatically decided by selecting theW/o ratio calculated to produce a hole ratio which conforms to the useand the purpose of the produced material.

(e) Polymerization Initiator

For the purpose of accomplishing the polymerization of an HIPE in a veryshort period of time as aimed at by this invention, it is advantageousto use a polymerization initiator. The polymerization initiator is onlyrequired to be suitable for use in the reversed phase emulsionpolymerization. It is not discriminated between the water-soluble typeand the oil-soluble type.

As concrete examples of the water-soluble polymerization initiator whichis used effectively herein, azo compounds such as2,2′-azobis(2-amidinopropane)dihydrochloride; persulfates such asammoniumpersulfate, potassiumpersulfate, and sodiumpersulfate; peroxidessuch as potassiumperacetate, sodiumperacetate, sodiumpercarbonate,potassiumperacetate may be cited. As concrete example of the oil-solublepolymerization initiator which is used effectively herein, peroxide suchas, cumene hydroperoxide, t-butyl hydroperoxide,t-butylperoxide-2-ethylhexyanoate di-t-butyl peroxide, diisopropylbenzene hydroperoxide, p-methane hydroperoxide, 1,1,3,3-tetramethylbutylhydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, benzoyl peroxide,and methylethyl ketone peroxide may be cited. These polymerizationinitiators may be used either singly or in the form of a combination oftwo or more members.

Combined use of two or more kinds of polymerization initiator havingdifferent 10 hour half period temperatures, i.e. the temperatures atwhich the concentrations of the relevant initiators are halved in 10hours proves advantageous. As a matter of course, it is permissible touse in combination the water-soluble polymerization initiator and theoil-soluble polymerization initiator.

The content of the polymerization initiator mentioned above is properlyin the range of 0.05-25 mass parts, preferably 1.0-10 mass parts, basedon 100 mass parts of the total mass of the monomer compositionconsisting of a polymerizing monomer and a cross-linking monomer, thoughit is variable with the combination of the polymer composition and thepolymerization initiator. If the content of the polymerization initiatoris less than 0.05 mass part, the shortage will be at a disadvantage inincreasing the amount of the unaltered monomer component andconsequently increasing the residual monomer content in the producedporous cross-linked polymer. Conversely, if the content of thepolymerization initiator exceeds 25 mass parts, the excess will be at adisadvantage in rendering the polymerization difficult to control anddegrading the mechanical property of the produced porous cross-linkedpolymer.

Alternatively, a redox polymerization initiator formed by combining thepolymerization initiator mentioned above with a reducing agent may beused. In this case, the polymerization initiator to be used herein doesnot need to be discriminated between the water-soluble type and theoil-soluble type. It is permissible to use a water-soluble redoxpolymerization initiator and an oil-soluble redox polymerizationinitiator in combination.

In the reducing agents, as concrete examples of the water-solublereducing agents, sodium hydrogen sulfite, potassium hydrogen sulfite,sodium thiosulfate, potassium thiosulfate, L-ascorbic acid, ferroussalts, formaldehyde sodiumsulfoxylate, glucose, dextrose, triethanolamine, and diathanol amine may be cited. As concrete examples of theoil-soluble reducing agent, dimethyl aniline, tin octylate, and cobaltnaphthenate may be cited. These redox polymerization initiator typereducing agents may be used either singly or in the form of a mixture oftwo or more members.

The ratio of the reducing agent contained in the redox polymerizationinitiator mentioned above (mass ratio), i.e. the polymerizationinitiator (oxidizing agent)/reducing agent, is in the approximate rangeof 1/0.01-1/10, preferably 1/0.2-1/5.

The polymerization initiator (inclusive of the redox polymerizationinitiator) is only required to be present at least during the course ofthe polymerization of an HIPE. It may be added {circle around (1)} tothe oil phase and/or the water phase prior to the formation of an HIPE,{circle around (2)} simultaneously with the formation of an HIPE, or{circle around (3)} after the formation of an HIPE. In the case of theredox polymerization initiator, the polymerization initiator (oxidizingagent) and the reducing agent may be added at different times.

(f) Salt

The salt as an arbitrary component for the composition of the HIPEmentioned above may be used when it is necessary for improving thestability of the HIPE.

As concrete examples of the salt of this nature, halogenides, sulfates,nitrates, and other similar water-soluble salts of alkali metals andalkaline earth metals such as calcium chloride, sodium sulfate, sodiumchloride, and magnesium sulfate may be cited. These salts may be usedeither singly or in the form of a combination of two or more members.Such a salt is preferred to be added in the water phase. Among saltsmentioned above, polyvalent metal salts prove particularly advantageousfrom the viewpoint of the stability of the HIPE during the course ofpolymerization.

The content of the salt mentioned above is proper in the range of 0.1-20mass parts, preferably 0.5-10 mass parts, based on 100 mass parts. Ifthe content of the salt exceeds 20 mass parts, the excess will be at adisadvantage in suffering the waste water squeezed out of the HIPE tocontain the water in an unduly large amount, boosting the cost for thedisposal of the waste water, failing to bring a proportional addition tothe effect, and not doing any good economically. If the content is lessthan 0.1 mass part, the shortage will possibly prevent the effect of theaddition of the salt from being fully manifested.

(g) Other Additive

Other various kinds of additives which are capable of improving theconditions of production, the property of HIPE, and the performance ofthe porous cross-linked polymer by imparting the performance and thefunction of their own, they may be suitably used herein. For example, abase and/or a buffer may be added for the purpose of adjusting the pHvalue. The content of the other additives may be selected within such arange that the additives used may fully manifest the performance,function, and further the economy commensurate with the purpose ofaddition. As such additives, activated carbon, inorganic powder, organicpowder, metallic powder, deodorant, antibacterial agent, antifungiagent, perfume and other highly polymerized compounds may be cited.

(2) Method for Preparation of HIPE

The method for production of the HIPE which can be used in thisinvention does not need to be particularly discriminated. Any of themethods for production of HIPE heretofore known to the art may besuitably used. A typical method for the production of interest will bespecifically described below.

First, a polymerizing monomer, a cross-linking monomer, and a surfactantas essential components and further an oil-soluble polymerizationinitiator (inclusive of an oil-soluble redox polymerization initiator)and other additives as optional components for the formation of an oilphase prepared in respectively specified amounts mentioned above arestirred at a prescribed temperature to produce a homogeneous oil phase.

Meanwhile, water as an essential component and further a water-solublepolymerization initiator (inclusive of a water-soluble redoxpolymerization initiator), salts, and other additives as optionalcomponents for the formation of a water phase prepared in respectivelyspecified amounts are stirred and heated to a prescribed temperature inthe range of 30-95° C. to produce a homogeneous water phase.

Then, the oil phase which is the mixture of the monomer component,surfactant, etc. and the water phase which is the mixture of water,water-soluble salt, etc., both prepared as described above are joined,mixed and stirred efficiently for exertion of proper shearing force andinduction of emulsification at the temperature for the formation of anHIPE (emulsifying temperature) which will be described specificallyhereinbelow to accomplish stable preparation of an HIPE. As a means forstirring and mixing the water phase and the oil phase particularly forthe table preparation of the HIPE, the method which comprises keepingthe oil phase stirred and continuously adding the water phase to thestirred oil phase over a period of several minutes to some tens ofminutes. Alternatively, the HIPE aimed at may be produced by stirringand mixing part of the water phase component and the oil phase componentthereby forming an HIPE resembling yogurt and continuing the stirringand mixing operation while adding the remaining portion of the waterphase component to the yogurt-like HIPE.

(3) Water Phase/Oil Phase (W/O) Ratio

The water phase/oil phase (W/O) ratio (by mass ratio) of the HIPEobtained as described above does not need to be particularlydiscriminated but may be properly selected, depending on the purpose ofuse of the porous cross-linked polymer possessed of open cells (such as,for example, water absorbent material, oil absorbent material, soundinsulation material, and filter). As defined above, it is properly notless than 3/1, preferably in the range of 10/1-250/1, and particularlypreferably in the range of 10/1-80/1.

(4) Apparatus for Production of HIPE

The apparatus for the production of the HIPE mentioned above does notneed to be particularly discriminated. Any of the apparatuses for theproduction of the porous cross-linked polymer material which have beenheretofore known to the art maybe used. For example, the stirring device(emulsifier) to be used for mixing and stirring the water phase and theoil phase may be selected from among the stirring devices and thekneading devices which have been heretofore known to the art. Asconcrete examples of the stirring device, stirring devices using vanesof the propeller type, the paddle type, and the turbine type,homomixers, line mixers, and pin mills may be cited.

(5) Temperature for Forming HIPE

The temperature for forming an HIPE is generally in the range of 20-110°C. From the viewpoint of the stability of the HIPE, the temperature ispreferably in the range of 30-105° C., more preferably 40-100° C. If thetemperature for forming the HIPE is less than 20° C., the shortage willpossibly result in unduly elongating the time for heating, depending onthe temperature of hardening. Conversely, if the temperature exceeds110° C., the excess will possibly result in degrading the stability ofthe formed HIPE. Incidentally, it is commendable to adjust preparatorilythe temperature of the oil phase and/or the water phase to theprescribed emulsifying temperature and then stir and mix the two phasestill emulsification and form the HIPE as expected. Since the preparationof the HIPE uses the water phase in a larger amount, the preparatoryadjustment of the temperature of at least the water phase to theprescribed emusifying temperature may well be rated as more favorable.If the polymerizing monomer or the cross-linking monomer begins topolymerize and forms a polymer while the emulsification is in progress,the formed polymer will possibly impair the stability of the HIPE. Whena polymerization initiator (inclusive of a redox polymerizationinitiator) is incorporated in the raw material for the preparation ofthe HIPE, therefore, the emulsifying temperature of the HIPE ispreferred to be incapable of inducing the polymerization initiator(oxidizing agent) to undergo substantial thermal decomposition enough toinitiate polymerization of the HIPE. More preferably, the emulsifyingtemperature is lower than the temperature at which the half-life of thepolymerization initiator (oxidizing agent) is 10 hours (10-hourhalf-life temperature).

(6) Formation of HIPE

The HIPE prepared as described above is formed in a necessary shapebefore the monomer in the HIPE is polymerized. In this invention, theHIPE to be formed can be supplied batchwise or continuously to the siteof formation. Here, the term “continuously” refers to the operation ofcontinuously supplying the produced HIPE to the forming device andsubsequently polymerizing it continuously in the polymerizing device andthe term “batchwise” refers to the operation of forming and polymerizingthe whole amount of the HIPE produced.

Since the continuous method which continuously forming and polymerizingthe HIPE enjoys high productivity and permits the dehydration thereof tobe continuously performed, it is at an advantage in enabling the methodof production contemplated by this invention to be utilized mosteffectively. Specifically, the method for continuous production of aporous cross-linked polymer sheet comprises continuously supplying anHIPE onto the belt of a running belt conveyor of such a construction asto heat the surface of the belt with a heating device and thensimultaneously forming and polymerizing the HIPE into a smooth sheet onthe belt. When the surface of the conveyor for contact with the emulsionis smooth, a continuous sheet of the polymer can be obtained in anecessary thickness by supplying the HIPE in a prescribed thickness ontothe belt.

The production of the porous cross-linked polymer in a three-dimensionalshape can be attained by cast polymerization, i.e. a method whichcomprises casting an HIPE into a female die of the shape mentioned aboveand polymerizing the HIPE in the female die. Incidentally, the castpolymerization may be implemented by the batchwise method mentionedabove or by the continuous method which uses the die adapted to remainin a continuous motion during the course of operation.

Properly, the HIPE is tightly sealed with a film of PET, for example, soas to keep the surface of the HIPE from contacting the ambient air. Thereason for the desirability of the tight seal is that the surface of theHIPE, on being exposed to the ambient air, can no longer maintain theporous thereof and the produced porous cross-linked polymer will bepossibly deficient in the water absorbing property. This tight seal isopened after the polymerization of the HIPE is completed.

(7) Polymerization of HIPE

(a) Addition of Polymerization Initiator

The HIPE begins to polymerize in consequence of the addition of apolymerization initiator and the application of heat. The polymerizationinitiator is added {circle around (1)} to the water phase and/or the oilphase and mixed therewith before the formation of an HIPE, {circlearound (2)} simultaneously with the formation of the HIPE, or {circlearound (3)} after the formation of the HIPE. In the case of {circlearound (2)}, the redox polymerization initiator system may be used forthe same reason as described above in {circle around (1)}, i.e. themethod for the formation of an HIPE.

In this case, the preliminary addition is conveniently made to the oilphase when the polymerization initiator or the reducing agent is solublein oil or to the water phase when it is soluble in water. A method ofadding to the water phase an emulsion of the oil-soluble polymerizationinitiator (oxidizing agent) or the reducing agent, for example, is alsoconceivable. The polymerization initiator may be used in an undilutedform or in the form of a solution or dispersion in water or an organicsolvent. When the addition is made simultaneously with or subsequentlyto the formation of an HIPE, it is commendable for the sake ofpreventing the monomer component from being polymerized unevenly toensure quick and uniform mixture of the added polymerization initiator.

The HIPE which has incorporated the polymerization initiator therein ispromptly introduced into a polymerization vessel which is apolymerization device or into a continuous polymerization device. It iscommendable to insert a route for introducing a reducing agent or anoxidizing agent or other polymerization initiator in a route emanatingfrom an emulsifier for preparing the HIPE and reaching a polymerizationvessel or a continuous polymerization device and add the polymerizationinitiator via the added route to the HIPE and mix them in a line mixer.

(b) Polymerization Temperature and Polymerization Time

The polymerization temperature of HIPE is generally in the range ofnormal room temperature to 150° C. and, from the standpoint of stabilityof the HIPE and the polymerization speed, preferably in the range of60-110° C., more preferably in the range of 80-110° C., and particularlypreferably in the range of 90-100° C. If the polymerization temperaturefalls short of the normal room temperature, the shortage will be at adisadvantage in requiring an unduly long time for the polymerization andpossibly rendering commercial production infeasible. Conversely, if thepolymerization temperature exceeds 150° C., the excess will be possiblyat a disadvantage in compelling the produced porous cross-linked polymerto acquire pores of a uniform diameter and betray deficiency instrength. The polymerization temperature may be varied in two stages orin more stages during the course of the polymerization. This inventiondoes not exclude the polymerization which is performed in this manner.

The polymerization time of an HIPE is generally in the range of oneminute to 20 hours, preferably within one hour, more preferably within30 minutes, and particularly preferably in the range of one to 20minutes. If the polymerization time exceeds 20 hours, the excess will bepossibly at a disadvantage commercially in degrading the productivity.Conversely, if the polymerization time falls short of one minute, theshortage will be possibly at a disadvantage in preventing the porouscross-linked polymer from acquiring fully satisfactory strength. Ofcourse, this invention does not exclude adoption of a polymerizationtime longer than the upper limit of the range mentioned above.

The polymer consequently obtained is cooled, possibly gradually, to anexpected temperature, though not necessarily optionally, the porouscross-linked polymer thus obtained may be subjected to the dehydration,the slicing, or the after treatment such as washing and compressionwithout being cooled. When the porous cross-linked polymer retainsmechanical strength enough to withstand the dehydration or the slicing,the polymerizing monomer contained in the RIPE does not need to havecompleted polymerization. In this case, the polymerization of themonomer may be attained by heating the porous cross-linked polymersubsequently to the dehydration or the slicing.

When the produced porous cross-linked polymer is to be sliced prior tothe dehydration into pieces of an expected thickness, any of the knowndevices heretofore adopted for the slicing under discussion may be usedin its unmodified form. Generally, the porous cross-linked polymer whichhas undergone the dehydration is sliced by rotating a saw tooth nippedbetween guides. The motion of the saw tooth does not need to be limitedto the rotation but may be produced longitudinally, laterally, orvertically. The guides may be absent in this case.

(c) Polymerization Device

The polymerization device which can be used in this invention does notneed to be particularly discriminated. A belt conveyor type continuouspolymerization device provided with a temperature adjusting unit or acontinuous casting type polymerizing device, for example, may be used.As described already in the paragraph covering the formation of an HIPE,the continuous method which simultaneously forms and polymerizes an HIPEcontinuously may utilize a device so constructed as to heat the surfaceof the belt of a belt conveyor with a heating device and adapted tosupply continuously the HIPE onto the belt in motion and form andpolymerize the HIPE into a smooth sheet on the belt. Naturally, a batchtype polymerization column or a batch type cast polymerization devicemay be used as occasion demands.

The polymerization of an HIPE in a batch operation is effected, forexample, by introducing the HIPE into a cylindrical polymerizationvessel and externally applying heat to the polymerization vessel. Thepurpose of obtaining the porous cross-linked polymer in the form ofsheet, however, is fulfilled by slicing a cylindrically formed porouscross-linked polymer in a prescribed thickness with a known peelingmachine thereby obtaining an elongate sheet in advance.

(d) Porous Cross-Linked Polymer

The thickness of the porous cross-linked polymer to be dehydrated inthis invention is preferably not more than 100 mm, more preferably inthe range of 0.5-50 mm, and particularly preferably in the range of0.5-30 mm. The reason for this range is that when the thickness is inthis range, the eventual water content can be lowered uniformly even tothe center of the sheet to a very bottom level as described above. Evenwhen the porous cross-linked polymer is produced by polymerizing an HIPEin a batch operation, therefore, it is sliced with the peeling machinein a thickness in the range of 0.5-100 mm.

The porous cross-linked polymer consequently obtained generally has awater content in the range of 300-25000 w/w %.

[III] Other Treatments for Porous Cross-Linked Polymer Sheet

The porous cross-linked polymer sheet of this invention is a sheet whichis obtained by the dehydration carried out by the method describedabove. Optionally, it may be subjected to some other treatments beforeor after the dehydration. The treatments which can be performed includea compressing treatment, washing treatment, drying treatment,impregnation treatment intended to impart prescribed characteristicproperties, and slicing treatment, for example. These treatments may beperformed at any proper stage after the preparation of the porouscross-linked polymer to suit the purpose.

(a) Washing Treatment

For the purpose of improving the surface condition of a porouscross-linked polymer sheet, for example, the porous cross-linked polymermay be washed with purified water or an aqueous solution containing anarbitrary additive or a solvent. The porous cross-linked polymer sheetwhich has been washed can be dehydrated by the method of dehydrationmentioned above. The water content eventually attained in the dehydratedsheet may be arbitrarily selected to suit the purpose for which thedehydrated sheet is used.

(b) Drying Treatment

The porous cross-linked polymer sheet produced by this invention, whennecessary, may be dried by being heated with hot wind or microwave andmay have the water content thereof adjusted by means of moistening.

(c) Impregnating Treatment

The porous cross-linked polymer sheet may be endowed with functionalityby the impregnating treatment using such additives as detergent,perfume, deodorant, and antifungal agent.

(d) Compressing Treatment

The porous cross-linked polymer sheet of this invention can becompressed into a form measuring one of several parts of the originalthickness. The porous cross-linked polymer compressed in the form of asheet has a small volume as compared with the original porouscross-linked polymer and permits a reduction in the cost oftransportation and storage. The porous cross-linked polymer in thecompressed form, on contacting a large volume of water, manifests thecharacter of absorbing water and resuming the original thickness. It ischaracterized by acquiring a faster water absorbing speed than thepolymer of the original thickness.

For the impartation of the compressed form, it suffices to use acompressing means which conforms to the shape of the porous cross-linkedpolymer to be compressed so as to exert pressure uniformly on the porouscross-linked polymer throughout the entire volume thereof and compressit evenly. Particularly, for the production of the porous cross-linkedpolymer in the form of sheet, it suffices to pass the slices cut fromthe polymer between rolls or belts which are opposed to each otheracross an interval adjusted in advance to a prescribed distance. At thestep of this compression, the temperature at which the porouscross-linked polymer is compressed is preferred to be higher than theglass transition temperature of the porous cross-linked polymer. If thistemperature is lower than the glass transition temperature of the porouscross-linked polymer, the shortage will possibly result in breaking theporous structure or varying the pore diameter. From the viewpoint ofsaving the space necessary for transportation and storage andfacilitating the ease of handling, it is effective to make thecompression to below one half of the original thickness. Preferably, thecompression is made to below ¼ of the original thickness. When the sheetof the porous cross-linked polymer at the end of the dehydrating stepacquires a thickness falling in the prescribed range, there is no needfor installing any new compresing step.

EXAMPLES

Now, this invention will be specifically described below with the aid ofworking examples.

Production Example 1

A fibrous sheet having a liquid absorbing power of 2.3 was obtained byfilling the gaps in a non-woven fabric formed by multidimensionallyintertwining 200 mass parts of polyester fibers having an extremelysmall thickness of 0.1 denier with a porous structure formed of 100 massparts of polyurethane. A multiplicity of discs punched out of thefibrous sheet were inserted side by side around a metallic shaftprovided with an aspiration mechanism to form a roll. The roll had theterminals thereof fixed by application of pressure of 50 kg/cm² and thesurface thereof subsequently polished to produce a non-woven fabric rollprovided with the aspiration mechanism. This non-woven fabric roll wasset in position in a dehydrating device.

Production Example 2

A fibrous sheet having liquid absorbing power of 5.5 was obtained byintertwining multidimensionally a mixture of 100 mass parts of 1-denierpolyester fibers and 100 mass parts of 5-denier polyester fibers. Amultiplicity of discs punched out of the fibrous sheet were insertedside by side around a metallic shaft provided with an aspirationmechanism to form a roll. The roll had the terminals thereof fixed byapplication of pressure of 50 kg/cm² and the surface thereofsubsequently polished to produce a non-woven fabric roll provided withthe aspiration mechanism. This non-woven fabric roll was set in place ina dehydrating device.

Production Example 3

A fibrous sheet having liquid absorbing power of 6.0 was obtained byintertwining multidimensionally a mixture of 100 mass parts of 1-denierpolyethylene terephthalate fibers and 100 mass parts of 5-denierpolyethylene terephthalate fibers. This fibrous sheet was wrapped aroundthe surface of a metallic shaft provided with an aspiration mechanism toproduce a non-woven fabric roll provided with the aspiration mechanism.This non-woven fabric roll was set in place in a dehydrating device.

Example 1

An oil phase was prepared by adding 0.4 mass part of diglycerinmonooleate to a mixture of 5.0 mass parts of 2-ethylhexyl acrylate and3.0 mass parts of 55% divinyl benzene and dissolving them altogetheruniformly. Separately, a water phase was prepared by dissolving 8.0 massparts of calcium chloride and 0.2 mass part of potassium persulfate in369.8 mass parts of purified water and then heated to 65° C. An HIPE wasformed by continuously supplying the oil phase and the water phase at aratio of 1/45 to a dynamic mixing device and mixed and emulsifiedtherein. This HIPE was introduced into a polymerization column andpolymerized therein at a temperature of 65° C. for 16 hours to obtain ashaped porous cross-linked polymer. This polymer was extracted from thepolymerization column and sliced into pieces 5 mm in thickness. Theslices were dehydrated under the conditions shown in Table 1. Theresults are shown in Table 1.

With reference to Table 1, in the bracket classifying the non-wovenfabric in kind, A denotes a sample of Production Example 1, B denotes asample of Production Example 2, C denotes a sample of Production Example3 respectively, D denotes a sample using opposed rolls each formed bywrapping a rubber sheet around a metallic roll (not provided with anaspiration mechanism), and E a sample using opposed stainless steelrolls including a lower roll which was provided therein with a suctionbox as an accessorial item.

Examples 2-4 and Comparative Examples 1-3

The porous cross-linked polymers obtained by the procedure of Example 1were dehydrated under the varying conditions shown in Table 1. Theresults are shown in Table 1.

The samples which used non-woven fabric in their dehydrating rollsinvariably showed satisfactory results as evinced by low final watercontents. In Comparative Examples 2 and 3, samples (E) using opposedstainless steel rolls including a lower roll provided therein with asuction box as an accessorial item and consequently adding to the numberof rolls and allowing further squeezing brought virtually no charge inthe final water contents.

TABLE 1 Comparative Example Example 1 2 3 4 1 2 3 W/O ratio 45/1 45/145/1 45/1 45/1 45/1 45/1 Water content 4500 4500 4500 4500 4500 45004500 (w/w %) Thickness of polymer 5 5 5 5 5 5 5 (mm) Kind of roll/numberof A/3 B/3 C/3 C/1 D/3 E/3 E/15 pairs of rolls A/2 Linear pressure ofroll 9 9 9 9 9 9 9 (kg/cm) Transferring speed 7 7 7 7 7 7 7 (m/min.)Final water content 90 110 240 100 1300 550 490 (w/w %)

Example 5

An oil phase was prepared by adding 0.4 mass part of diglycerinmonoleate to a mixture of 5.0 mass parts of 2-ethylhexylacrylate and 3.0mass parts of 55% divinylbenzene and dissolving them altogetheruniformly. Separately, a water phase was prepared by dissolving 8.0 massparts of calcium chloride and 0.2 mass part of potassium persulfate in285.8 mass parts of purified water and then heated to 65° C. An HIPE wasformed by continuously supplying the oil phase and the water phase at aratio of 1/35 to a dynamic mixing device and mixed and emulsifiedtherein.

This HIPE was polymerized by a continuous polymerization devicecomprising an endless steel belt and an upper and a lower PET film at atemperature of 95° C. for 10 minutes at a speed of 7 m/min and formed ina shape 35 mm in thickness. Then, the upper and lower PET films wereseparated by a reeling motion. The denuded polymer layer was dehydratedunder the conditions shown in Table 2. The results are shown in Table 2.In Table 2, the classification of rolls by kind is made in the samemanner as in Table 1.

Examples 6-8 and Comparative Example 4 and 5

The porous cross-linked polymers obtained by the procedure of Example 5were dehydrated under the varying conditions shown in Table 2. Theresults are shown in Table 2.

The samples of Examples 5-8 which used non-woven fabric rolls invariablyeffected dehydration with high efficiency. In contrast, the samples ofComparative Examples 4 and 5 which used no non-woven fabric rollrequired rolls in a large number and showed only low final watercontents.

TABLE 2 Comparative Example Example 5 6 7 8 4 5 W/O ratio 35/1 35/1 35/135/1 35/1 35/1 Water content (w/w %) 3500 3500 3500 3500 3500 3500Thickness of polymer 35 35 35 35 35 35 (mm) Kind of roll/number A/5 B/5C/6 C/3 D/30 E/30 of pairs of rolls A/3 Transferring speed 7 7 7 7 7 7(m/min.) Final water content 250 310 390 260 1000 720 (w/w %)

Examples 9 and 10 and Comparative Example 6

An oil phase was prepared by adding 0.4 mass part of diglycerinmonooleate to a mixture of 5.0 mass parts of 2-ethylhexyl acrylate and3.0 mass parts of 55% divinyl benzene and dissolving them altogetheruniformly. Separately, a water phase was prepared by dissolving 8.0 massparts of calcium chloride and 0.2 mass part of potassium persulfate in453.8 mass parts of purified water and then heated to 65° C. An HIPE wasformed by continuously supplying the water phase and the oil phase at aratio of 65/1 to a dynamic mixing device and mixed and emulsifiedtherein.

This HIPE was polymerized by a continuous polymerization devicecomprising an endless steel belt and an upper and a lower PET film at atemperature of 95° C. for 10 minutes at a speed of 15 m/min and formedin a shape 10 mm in thickness. Then, the upper and lower PET films wereseparated by a reeling motion. The denuded polymer layers were graduallysqueezed with varying dehydrating rolls shown in Table 3 to compare thefinal water contents. The results are shown in Table 3. In Table 3, theclassification of rolls by kind is made in the same manner as in Table1.

In the dehydration of a porous cross-linked polymer which had a high W/Oratio and was exceedingly susceptible of collapse, when the porouscross-linked polymer was squeezed without interposing a mesh beltbetween the porous cross-linked polymer and the dehydrating rolls, itwas necessary to effect this squeezing gradually, increase the number ofrolls required to be used, and keep the operating speed of the squeezingline from being increased for the purpose of preventing the porouscross-linked polymer from breaking. The samples having a mesh beltinterposed between the porous cross-linked polymer and the dehydratingrolls permitted the squeezing to proceed efficiently without entailingoccurrence of a crack or a break and attained satisfactorily low finalwater contents.

TABLE 3 Comparative Example Example 9 10 6 W/O ratio 65/1 65/1 65/1Water content (w/w%) 6500 6500 6500 Thickness of polymer (mm) 10 10 10Presence/absence of mesh None Used None Kind of roll/number of A/7 A/5A/5 pairs of rolls Transferring speed 0.5 15 15 (m/min.) Final watercontent 240 250 — (w/w %) Remark No crack No crack Crack occurred

Examples 11-13 and Comparative Examples 7 and 8

An oil phase was prepared by adding 0.4 mass part of diglycerinmonooleate to a mixture of 5.0 mass parts of 2-ethylhexyl acrylate and3.0 mass parts of 55% divinyl benzene and dissolving them altogetheruniformly. Separately, a water phase was prepared by dissolving 8.0 massparts of calcium chloride and 0.2 mass part of potassium persulfate in369.8 parts of purified water and then heated to 65° C. An HIPE wasformed by continuously supplying the water phase and the oil phase at aratio of 45/1 to a dynamic mixing device and mixed and emulsifiedtherein. This HIPE was polymerized by a continuous polymerization devicecomprising an endless steel belt and an upper and a lower PET film at atemperature of 95° C. for 10 minutes at a speed of 10 m/min and formedin a shape 10 mm in thickness. Then, the upper and lower PET films wereseparated by a reeling motion. The denuded polymer layers were squeezedwith varying dehydrating rolls shown in Table 4. The results are shownin Table 4. In Table 4, the classification of rolls by kind is made inthe same manner as in Table l. The durability was rated on a three-pointscale, wherein ◯ denotes the absence of a discernible sign of decline inthe dehydrating capacity due to continuous operation, Δ denotes a slightdecline of the dehydrating capacity due to continuous operation to anextent of necessitating backwash, and x denotes occurrence of cloggingdue to continuous operation.

The results indicate that the combination of preliminary squeezing andnon-woven fabric rolls lowered final water content and impartedoutstanding durability to the non-woven fabric rolls.

TABLE 4 Example Comparative Example 11 12 13 7 8 W/O ratio 45/1 45/145/1 45/1 45/1 Water content 4500 4500 4500 4500 4500 (w/w %) Thicknessof  10  10  10  10  10 polymer (mm) Preliminary squeezing Kind of E/15E/15 — E/15 E/15 roll/number of pairs of rolls Mesh sheet Attached toAttached to — None Attached to all rolls all rolls all rollsTransferring  10  10 — 10  10 speed (m/min.) Final water 1000 1000 — —1000 content (w/w %) (Crack occurred) Dehydrating treatment Kind of A/3B/3 A/5 — — roll/number of pairs of rolls Mesh sheet None None Attached— — up to 3rd roll Transporting  10  10  10 — — speed (m/min) Finalwater 180 240 190 — — content (w/w %) Durability ◯ ◯ Δ — ◯ rated

Examples 14-16 and Comparative Example 9

An oil phase was prepared by adding 0.4 mass part of diglycerinmonooleate to a mixture of 5.0 mass parts of 2-ethylhexyl acrylate and3.0 mass parts of 55% divinyl benzene and dissolving them altogetheruniformly. Separately, a water phase was prepared by dissolving 8.0 massparts of calcium chloride and 0.2 mass part of potassium persulfate in411.8 parts of purified water and then heated to 65° C. An HIPE wasformed by continuously supplying the water phase and the oil phase at aratio of 55/1 to a dynamic mixing device and mixed and emulsifiedtherein. This HIPE was polymerized by a continuous polymerization devicecomprising an endless steel belt and an upper and a lower PET film at atemperature of 95° C. for 10 minutes at a speed of 10 m/min and formedin a shape 25 mm in thickness. Then, the upper and lower PET films wereseparated by a reeling motion. The denuded polymer layers were squeezedwith varying dehydrating rolls shown in Table 5. The results are shownin Table 5. In Table 5, the classification of rolls by kind is made inthe same manner as in Table 1 and the durability is rated in the samemanner as in Table 4. The results indicate that the combination ofpreliminary squeezing and non-woven fabric rolls lowered final watercontent and imparted outstanding durability to the non-woven fabricrolls.

TABLE 5 Comparative Example Example 14 15 16 9 W/O ratio 55/1 55/1 55/155/1 Water content 5500 5500 5500 5500 (w/w %) Thickness of polymer 2525 25 25 (mm) uz,12/21 Preliminary squeezing Kind of roll/number E/30E/30 — E/30 of pairs of rolls Mesh sheet Attached Attached — Attached toall to all to all rolls rolls rolls Transferring speed 10 10 — 10(m/min.) Final water content 1100 1100 1100 (v/v%) Dehydrating treatmentKind of roll/number of A/5 B/5 A/6 — pairs of rolls Mesh sheet None NoneAttached — up to 3rd roll Transporting speed 10 10 10 — (m/min.) Finalwater content 190 290 220 (w/w %) Durability rated ◯ ◯ Δ ◯

INDUSTRIAL APPLICABILITY

In accordance with this invention, the use of non-woven fabric rollsfurnished with an aspiration mechanism enables a porous cross-linkedpolymer to be dehydrated to an extremely low water content. In order forthe conventional method to acquire this low water content, it generallynecessitates an increase in the linear pressure of roll and consequentlyinduces the porous cross-linked polymer to sustain fracture.

What is claimed is:
 1. A method for the production of a porouscross-linked polymer sheet, comprising the steps of forming andpolymerizing a water-in-oil type high internal phase emulsion therebyobtaining a porous cross-linked polymer and dehydrating the porouscross-linked polymer by using a non-woven fabric roll furnished with anaspiration mechanism.
 2. A method according to claim 1, wherein thenon-woven fabric used in said non-woven fabric roll possesses a liquidabsorbing power of not less than 1.0.
 3. A method according to claim 1,wherein the porous cross-linked polymer is subjected to preliminarysqueezing and thereafter to the dehydration by the use of a non-wovenfabric roll furnished with an aspiration mechanism.
 4. A methodaccording to claim 1, wherein a mesh belt is interposed between theporous cross-linked polymer and the dehydrating roll during thedehydration by the use of a non-woven fabric roll furnished with anaspiration mechanism or during the preliminary squeezing or subsequentdehydration by the use of a non-woven fabric roll furnished with anaspiration mechanism.
 5. A method according to claim 2, wherein theporous cross-linked polymer is subjected to preliminary squeezing andthereafter to the dehydration by the use of a non-woven fabric rollfurnished with an aspiration mechanism.
 6. A method according to claim2, wherein a mesh belt is interposed between the porous cross-linkedpolymer and the dehydrating roll during the dehydration by the use of anon-woven fabric roll furnished with an aspiration mechanism or duringthe preliminary squeezing or subsequent dehydration by the use of anon-woven fabric roll furnished with an aspiration mechanism.
 7. Amethod according to claim 3, wherein a mesh belt is interposed betweenthe porous cross-linked polymer and the dehydrating roll during thedehydration by the use of a non-woven fabric roll furnished with anaspiration mechanism or during the preliminary squeezing or subsequentdehydration by the use of a non-woven fabric roll furnished with anaspiration mechanism.
 8. A method according to claim 6, wherein theporous cross-linked polymer is subjected to preliminary squeezing andthereafter to the dehydration by the use of a non-woven fabric rollfurnished with an aspiration mechanism.